1. Field of the Invention
The present invention relates generally to a microwave detector for detecting a microwave transmitted from a specific source, such as a measurement equipment. More specifically, the invention relates to a microwave detector which may reduce power consumption by intermittent operation of a receiver circuit.
2. Description of the Related Art
Multi-band microwave detectors, such as so-called radar detectors which are adapted to detect microwaves transmitted from radar type speed measurement alarm equipment, have been known in the prior art. For example, U.S. Pat. No. 4,622,553, issued on Nov. 11, 1986 to Baba et al., U.S. Pat. No. 4,630,054, issued on Dec. 16, 1986 to Martinson, U.S. Pat. No. 4,954,828, issued on Sep. 4, 1990 to Orr disclose radar detectors of the type that the present invention is directed. Typical traffic monitoring radar type speed measuring equipment (hereinafter referred to as "radar") employ microwave in a plurality of frequency bands, i.e. 10 GHz band (X band), 24 GHz band (K band) and 35 GHz band (Ka band). Typically, there are two types of the radars employed for traffic speeding control. One type of the radar employs a continuous wave type speed measuring equipment which continuously transmit the microwave. The other radar employs a pulse-modulation type speed measuring equipment which transmits a pulse-modulated microwave.
A microwave detector, so-called radar detector, typically employs a super heterodyne type receiver circuit for receiving microwaves and discriminating a specific microwave in a specific band transmitted from the radar for generating an alarm by way of turning ON of a buzzer or LED indicator or so forth. In the case of the normal type microwave detector which is powered by an external power source, the super-heterodyne receiver circuit and other signal processing circuits may operate constantly without fear of exhausting the power source. Therefore, in this case, sweeping of an output frequency of a local oscillator is performed continuously in a cyclic manner. On the other hand, in the case of a battery powered microwave detector employing an installed battery or a solar battery, sweeping of the output frequency of the local oscillator is performed intermittently with a given interval in order to save power consumption.
As can be naturally understood, the power consumption will become smaller by employing a longer interval between the output frequency sweeping periods in the microwave detector. In the typical power-saving type microwave detector, a 10 msec active period is provided with a 490 msec resting interval. Therefore, the typical microwave detector will repeat the 10 msec active period and 490 msec resting interval in a cyclic manner. With such an operation timing, there is no problem in detecting a continuous, non-modulated microwave. On the other hand, it may significantly lower a detecting certainty for a pulse-modulated microwave transmitted from the pulse-modulation type radar, which pulse-modulated microwave typically has a pulse width of 50 msec. Namely, since the 1 0 msec active period is repeated with every 500 msec, a certainty of overlapping of the antenna input and the active period of the microwave detector becomes substantially low.
In order to certainly detect the pulse-modulated microwave with 50 msec of pulse width, it becomes necessary to set the period of cyclic operation smaller than or equal to 50 msec. However, on the other hand, when the resting interval is shortened for achieving the cyclic period shorter than or equal to 50 msec with maintaining the 10 msec active period, the power to be consumed is significantly increased. On the other hand, in order to maintain the power consumption level comparable to the timing of 10 msec active state with 490 msec resting interval, the active period has to be about 1 msec.
In case of the multi-band microwave detector, it is required to sweep the frequency of the oscillator within a predetermined frequency range within every active period in order to achieve desired reception band width. Reduction of the active period from 10 msec to 1 msec thus means to accelerate the sweeping speed at a 10 times higher speed. However, since the sweeping speed is limited by the characteristics of an intermediate frequency filter in the super-heterodyne receiver circuit, the characteristics of a filter in an audio stage following detection output. Namely, when the sweep speed is set at an excessively higher speed, the reception sensitivity may be lowered in inverse proportion to the sweeping speed.
In general, the output level of the pulse-modulation type radar is high, and while the output level of the continuous wave type speed radar is low. If the sweeping speed is set at an extremely high speed to lower the reception sensitivity, the detecting sensitivity for the non-modulated continuous microwave becomes unacceptably low to degrade reliability while the pulse-modulated microwave can be detected for a high level.